In a typical laser printer, radiation from laser is shaped, and imaged onto the film plane to produce the desired spot size. The spot forms the smallest element of the image, called "pixel". The laser radiation is modulated in an imagewise fashion to create right image density, pixel by pixel. The laser spot is scanned in line and page directions to create a two-dimensional image. If a continuous wave (CW) type gas or solid state laser is used, an external modulator of Acousto-Optic or Electro-Optic type is favored; for semiconductor diode lasers, laser radiation is modulated by varying the current input to the laser. For printers using high sensitivity media such as silver halide, electrophotoconductor, etc., high throughput is obtained by scanning the laser beam in the line direction by a rotating polygonal mirror, a galvanometer or a hologonal diffractive element, also called "flying spot" printers. For low sensitivity media printer such as a laser thermal printer, higher power lasers are used and exposure requirements of the order of 0.5 joules/cm.sup.2 are met by scanning the laser beam slowly in both line and page directions. One way to achieve this is to configure the printer like a "lathe", where line scan is obtained by a rotating drum which holds the film and page scan by translating the laser or drum in the direction along the axis of rotation. For higher throughput, higher power levels are required which cannot be met by a single laser technologically. Many discrete lasers are then ganged together to form multiple spots on the film plane. Multiple pixels are written simultaneously to increase the throughput. An architecture of such a printer using many discrete lasers coupled to optical fibers is suggested in U.S. Pat. No. 4,911,526 and assigned to the same assignee. The cost of discreet lasers and loss of efficiency in coupling to fibers has prompted yet another improvement on this basic concept of multiple lasers, which uses a monolithic array of lasers. The elements of the array are imaged directly on the film to produce multiple spots. Elements of the array are individually modulated to obtain pixel densities. An architecture of such a device is suggested in U.S. Pat. No. 4,804,975 and assigned to the same assignee. The problem with such a device is the complexity of fabricating an array where elements can be individually modulated and the need to modulate high current input to each element at fairly high speeds. The current driver electronics becomes expensive and difficult. High laser power capacity of each element makes it more susceptible for thermal and electrical cross-talks, which create image artifacts. Schemes to eliminate thermal and electrical cross-talks are difficult and expensive. Failure of even one element in the array makes it useless. Planarity and accurate mutual configuration of these elements are essential for use of reasonable optical system and artifact free images.
U.S. Pat. No. 5,132,723 is directed to a method and apparatus for controlling exposure using light valves. The light valve is being imaged onto an object, for example, a sheet of light-sensitive material and the image is scanned along the object either by moving the object or the image. Such an arrangement allows for the correction of exposure and/or compensation for any dead sites. However, such a practice clearly reduces the total throughput of data and requires complicated, individualized calibration techniques to compensate and adjust the exposure for each site in the array.
U.S. Pat. No. 5,049,901 relates to a light modulator that uses large area light sources such as an arc lamp to image onto a two-dimensional light valve, preferably of the deformable mirror type. The light valve is imaged onto a light-sensitive material and the image is scanned along the light-sensitive material. Because the light source is a broad spectrum source, the light valve will absorb energy near the fringes that will result in elevating the operating temperature of the light valve. Because the medium has a limited absorption band energy in that band, it will be used for writing, but a substantial portion of that energy will be absorbed by the light valve. The preferred way of operation requires that a substantial amount of the energy available in a light source should be used for writing purposes and not to raise the temperature of the equipment and surroundings.
U.S. Pat. No. 5,208,818 is directed to a laser system for recording data patterns on a planar substrate. A deformable mirror spatial light modulator is used together with a waveguide-type excimer laser to record a large number of data bits with each laser pulse. This is an ultraviolet (UV) pulsed excimer laser and uses a medium having a thin ablatable coating on a quartz substrate. Multiple exposures by the pulsed laser is used on the modulator to reduce average power density on the modulator. Such a system can find use in the manufacture of circuit boards using photoresist. Contrary to this technique, the present invention uses a constant wave (CW) source for thermal transfer or removal of thermal dyes to record images.
This invention provides solutions which overcome the above-mentioned problems. Laser or laser array is used as the illumination source. A reflective or transmittive modulator is illuminated uniformly by the light from all elements of laser. The elements of the modulator break up the light beam in image elements. Each element of the modulator is subsequently imaged on the film plane to form desired size spots. The modulator can be of a linear or area type with a variety of element densities. Examples of modulators are PLZT, liquid crystal, deflecting, deformable mirrors and elastomers, etc. Modulation characteristics of each modulator element enables image pixel density.